Golf ball

ABSTRACT

An object of the present disclosure is to provide a golf ball having excellent durability and flight distance and good shot feeling for an average golfer who hits a golf ball at a slow head speed. The present disclosure provides a golf ball comprising a spherical core and at least two cover layers covering the spherical core, wherein a difference between a core surface crosslinking density and a core center crosslinking density is more than 1.0×10 2  mol/m 3  and less than 9.0×10 2  mol/m 3 , a hardness difference between a core surface hardness Cs (Shore C hardness) and a core center hardness Co (Shore C hardness) is 13.0 or more and 30.0 or less, a compression deformation amount of the core when applying a load from an initial load of 98 N to a final load of 1275 N to the core is less than 3.8 mm, and the at least two cover layers include a first cover layer and a second cover layer positioned closer to the spherical core than the first cover layer, an average hardness Dave=(Ti×Hi+To×Ho)/(Ti+To) of the first cover layer and the second cover layer is 55 or more, wherein To (mm) is a thickness of the first cover layer, Ho (Shore D) is a slab hardness of the first cover layer, Ti (mm) is a thickness of the second cover layer, and Hi (Shore D) is a slab hardness of the second cover layer.

FIELD OF THE DISCLOSURE

The present disclosure relates to a golf ball, and more specificallyrelates to a technology for improving a core and a cover of a golf ball.

DESCRIPTION OF THE RELATED ART

Golf is a sport that can be played by golfers of all ages and genders.There are golfers with various skill levels. A so-called high-skilledgolfer hits a golf ball at a high head speed and thus uses a relativehard golf ball. A golf ball is compressed firmly when the golf ball ishit at a high head speed, thus the initial speed of the golf ballbecomes high. A golf ball having a high initial speed travels a greatflight distance. In addition, if the golfer with a high head speed hitsa soft golf ball, good shot feeling is obtained. On the other hand, anexcessively hard golf ball has lowered durability.

A golf ball is generally composed of a spherical core and a covercovering the spherical core. As a material for forming a core of a golfball, a rubber composition containing a base rubber, a co-crosslinkingagent and a crosslinking initiator is widely used in light of its goodresilience.

For example, JP 2015-077405 A discloses a golf ball comprising a coreand a cover composed of at least one layer, wherein letting R (mm) be aradius of the core, A be a JIS-C hardness at a center of the core, B bea JIS-C hardness at a position R/3 mm away from the core center, C be aJIS-C hardness at a position R/1.8 mm away from the core center, D be aJIS-C hardness at a position R/1.3 mm away from the core center, and Ebe a JIS-C hardness at a surface of the core, the core has across-sectional hardness which satisfies formulas (1) to (4) below:D−C≤7  (1)C−B≤7  (2)(D−C)−(C−B) and  (3)E−A≥16.  (4)

In addition, JP 2015-047502 A discloses a golf ball comprising a coreand a cover composed of one layer or a plurality of layers, wherein thecore is formed from a heat-molded product of a rubber compositioncontaining the following components (A) to (C):

-   -   (A) a base rubber,    -   (B) an organic peroxide, and    -   (C) water and/or a metal monocarboxylate; and        wherein a product (P×E) of a difference P (mol/m³) in a        crosslinking density between a surface of the core and a center        of the core measured based on a toluene swelling test,        multiplied by a deflection E (mm) of the core when applying an        initial load of 98N (10 kgf) to a final load of 1275 N (130 kgf)        is 28×10² mol/m³·mm or more.

JP 2001-149504 A discloses a one-piece golf ball composed of a ball bodyformed from a white rubber composition containing a base rubber, anα,β-unsaturated carboxylic acid and/or a metal salt thereof and anorganic peroxide, and a clear coat painted on a surface of the ballbody, wherein the white rubber composition contains 0.1 to 5.0 parts bymass of an antioxidant and 0.05 to 3.0 parts by mass of a lightstabilizer with respect to 100 parts by mass of the base rubber, and theclear coat contains 0.05 to 5.0 parts by mass of an UV absorber withrespect to 100 parts by mass of a resin component.

JP 2004-121815 A discloses a golf ball comprising a heat-molded productformed from a rubber composition as a constituent element, wherein therubber composition contains a polybutadiene including a cis-1,4 bond inan amount of 60 weight % or more and having a Mooney viscosity(M₁₊₄(100° C.)) of 40 or more, an unsaturated carboxylic acid and/or ametal salt thereof, an organic peroxide, and a monophenol-basedantioxidant.

JP S63-212377 A discloses a solid golf ball comprising an elasticportion formed from a rubber composition containing a base rubber, anα,β-ethylenically unsaturated carboxylic acid metal salt, anethylenically unsaturated carboxylic acid ester having a hindered phenolgroup, and a peroxide as at least one part of the solid golf ball.

SUMMARY OF THE DISCLOSURE

An object of the present disclosure is to provide a golf ball havingexcellent durability and flight distance and good shot feeling for agolfer with a high head speed.

The present disclosure provides a golf ball comprising a spherical coreand at least two cover layers covering the spherical core, wherein thespherical core is formed from a core rubber composition containing (a) abase rubber, (b) an α,β-unsaturated carboxylic acid and/or a metal saltthereof as a co-crosslinking agent, and (c) a crosslinking initiator, adifference (core surface crosslinking density−core center crosslinkingdensity) between a surface crosslinking density of the spherical coreand a center crosslinking density of the spherical core is more than1.0×10² mol/m³ and less than 9.0×10² mol/m³, a hardness difference(Cs−Co) between a surface hardness Cs (Shore C hardness) of thespherical core and a center hardness Co (Shore C hardness) of thespherical core is 13.0 or more and 30.0 or less, a compressiondeformation amount of the spherical core when applying a load from aninitial load of 98 N to a final load of 1275 N to the spherical core isless than 3.8 mm, and the at least two cover layers include a firstcover layer and a second cover layer positioned closer to the core thanthe first cover layer, an average hardness Dave=(Ti×Hi+To×Ho)/(Ti+To) ofthe first cover layer and the second cover layer is 55 or more, where To(mm) is a thickness of the first cover layer, Ho (Shore D) is a slabhardness of the first cover layer, Ti (mm) is a thickness of the secondcover layer, and Hi (Shore D) is a slab hardness of the second coverlayer.

The golf ball according to the present disclosure has excellentdurability and great flight distance and good shot feeling for a golferwith a high head speed by having the above configuration.

According to the present disclosure, a golf ball having excellentdurability and great flight distance and good shot feeling for a golferwith a high head speed is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a partially cutaway cross-sectional view of a golf ballaccording to one embodiment of the present disclosure.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present disclosure provides a golf ball comprising a spherical coreand at least two cover layers covering the spherical core, wherein thespherical core is formed from a core rubber composition containing (a) abase rubber, (b) an α,β-unsaturated carboxylic acid and/or a metal saltthereof as a co-crosslinking agent, and (c) a crosslinking initiator, adifference (core surface crosslinking density−core center crosslinkingdensity) between a surface crosslinking density of the spherical coreand a center crosslinking density of the spherical core is more than1.0×10² mol/m³ and less than 9.0×10² mol/m³, a hardness difference(Cs−Co) between a surface hardness Cs (Shore C hardness) of thespherical core and a center hardness Co (Shore C hardness) of thespherical core is 13.0 or more and 30.0 or less, a compressiondeformation amount of the spherical core when applying a load from aninitial load of 98 N to a final load of 1275 N to the spherical core isless than 3.8 mm, and the at least two cover layers include a firstcover layer and a second cover layer positioned closer to the sphericalcore than the first cover layer, an average hardnessDave=(Ti×Hi+To×Ho)/(Ti+To) of the first cover layer and the second coverlayer is 55 or more, where To (mm) is a thickness of the first coverlayer, Ho (Shore D) is a slab hardness of the first cover layer, Ti (mm)is a thickness of the second cover layer, and Hi (Shore D) is a slabhardness of the second cover layer.

The spherical core of the golf ball according to the present disclosureis formed from a core rubber composition containing (a) a base rubber,(b) an α,β-unsaturated carboxylic acid and/or a metal salt thereof as aco-crosslinking agent, and (c) a crosslinking initiator. First,materials used for the spherical core of the golf ball according to thepresent disclosure will be explained.

[(a) Base Rubber]

As (a) the base rubber, a natural rubber and/or a synthetic rubber isused. For example, a polybutadiene rubber, a natural rubber, apolyisoprene rubber, a styrene polybutadiene rubber, or anethylene-propylene-diene rubber (EPDM) can be used. These rubbers may beused solely, or at least two of these rubbers may be used incombination. Among them, particularly preferred is a high-cispolybutadiene having a cis-1,4 bond in an amount of 40 mass % or more,preferably 80 mass % or more, more preferably 90 mass % or more, andeven more preferably 95 mass % or more in view of their superiorresilience.

From the viewpoint of obtaining a core having higher resilience, theamount of the high-cis polybutadiene in the base rubber is preferably 60mass % or more, more preferably 80 mass % or more, and even morepreferably 90 mass % or more. It is also preferable that (a) the baserubber consists of the high-cis polybutadiene.

The high-cis polybutadiene preferably has a 1,2-vinyl bond in an amountof 2.0 mass % or less, more preferably 1.7 mass % or less, and even morepreferably 1.5 mass % or less. If the amount of the 1,2-vinyl bond isexcessively high, the resilience may be lowered.

The high-cis polybutadiene preferably includes a polybutadienesynthesized using a rare earth element catalyst. When a neodymiumcatalyst, which employs a neodymium compound that is a lanthanum seriesrare earth element compound, is used, a polybutadiene rubber having ahigh content of a cis-1,4 bond and a low content of a 1,2-vinyl bond isobtained with excellent polymerization activity. Such a polybutadienerubber is particularly preferred.

The high-cis polybutadiene preferably has a Mooney viscosity (ML₁₊₄(100°C.)) of 30 or more, more preferably 32 or more, even more preferably 35or more, and preferably has a Mooney viscosity (ML₁₊₄(100° C.)) of 140or less, more preferably 120 or less, even more preferably 100 or less,and most preferably 55 or less. It is noted that the Mooney viscosity(ML₁₊₄(100° C.)) in the present disclosure is a value measured accordingto JIS K6300 using an L rotor under the conditions of: a preheating timeof 1 minute; a rotor revolution time of 4 minutes; and a temperature of100° C.

The high-cis polybutadiene preferably has a molecular weightdistribution Mw/Mn (Mw: weight average molecular weight, Mn: numberaverage molecular weight) of 2.0 or more, more preferably 2.2 or more,even more preferably 2.4 or more, and most preferably 2.6 or more, andpreferably has a molecular weight distribution Mw/Mn of 6.0 or less,more preferably 5.0 or less, even more preferably 4.0 or less, and mostpreferably 3.0 or less. If the molecular weight distribution (Mw/Mn) ofthe high-cis polybutadiene is excessively low, the processabilitydeteriorates. If the molecular weight distribution (Mw/Mn) of thehigh-cis polybutadiene is excessively high, the resilience may belowered. It is noted that the measurement of the molecular weightdistribution is conducted by gel permeation chromatography(“HLC-8120GPC”, available from Tosoh Corporation) using a differentialrefractometer as a detector under the conditions of column: GMHHXL(available from Tosoh Corporation), column temperature: 40° C., andmobile phase: tetrahydrofuran, and the molecular weight distribution iscalculated by converting based on polystyrene standard.

[(b) Co-Crosslinking Agent]

(b) The α,β-unsaturated carboxylic acid having 3 to 8 carbon atomsand/or the metal salt thereof used for the rubber composition is blendedas a co-crosslinking agent in the rubber composition, and has an actionof crosslinking a rubber molecule by graft polymerization to a baserubber molecular chain.

Examples of the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms include acrylic acid, methacrylic acid, fumaric acid, maleic acidand crotonic acid.

Examples of the metal ion constituting the metal salt of theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms include amonovalent metal ion such as sodium, potassium and lithium; a divalentmetal ion such as magnesium, calcium, zinc, barium and cadmium; atrivalent metal ion such as aluminum; and other metal ion such as tinand zirconium. The above metal component may be used solely or as amixture of at least two of them. Among them, the divalent metal ion suchas magnesium, calcium, zinc, barium and cadmium is preferably used asthe metal component. This is because if the divalent metal salt of theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms is used, ametal crosslinking easily generates between the rubber molecules.Especially, as the divalent metal salt, zinc acrylate is preferable,because use of zinc acrylate enhances the resilience of the obtainedgolf ball. It is noted that the α,β-unsaturated carboxylic acid having 3to 8 carbon atoms and/or the metal salt thereof may be used solely or asa mixture of at least two of them.

The amount of (b) the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms and/or the metal salt thereof is preferably 20 parts bymass or more, more preferably 25 parts by mass or more, and even morepreferably 30 parts by mass or more, and is preferably 55 parts by massor less, more preferably 50 parts by mass or less, and even morepreferably 45 parts by mass or less, with respect to 100 parts by massof (a) the base rubber. If the amount of (b) the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereofis less than 20 parts by mass, the amount of (c) the crosslinkinginitiator which will be described later must be increased such that thecured product (e.g. core) formed from the rubber composition has anappropriate hardness, which tends to lower the resilience of theobtained golf ball. On the other hand, if the amount of theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or themetal salt thereof is more than 55 parts by mass, the cured product(e.g. core) formed from the rubber composition becomes so hard that theshot feeling of the obtained golf ball may be lowered.

[(c) Crosslinking Initiator]

(c) The crosslinking initiator used for the rubber composition isblended to crosslink (a) the base rubber component. As (c) thecrosslinking initiator, an organic peroxide is suitable. Specificexamples of the organic peroxide include dicumyl peroxide,1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy) hexane and di-t-butyl peroxide. Theseorganic peroxides may be used solely or as a mixture of at least two ofthem. Among them, dicumyl peroxide is preferably used.

In case that the rubber composition contains at least one type of thefollowing components (d), the amount of (c) the crosslinking initiatoris preferably 0.2 part by mass or more, more preferably 0.5 part by massor more, and even more preferably 0.7 part by mass or more, and ispreferably 5.0 parts by mass or less, more preferably 2.5 parts by massor less, and even more preferably 2.0 parts by mass or less, withrespect to 100 parts by mass of (a) the base rubber. If the amount ofthe crosslinking initiator is less than 0.2 part by mass, the curedproduct (e.g. core) formed from the rubber composition is so soft thatthe resilience of the obtained golf ball tends to be lowered, and if theamount of the crosslinking initiator is more than 5.0 parts by mass, theamount of (b) the co-crosslinking agent described above must bedecreased such that the cured product (e.g. core) formed from the rubbercomposition has an appropriate hardness, which tends to lower theresilience or worsen the durability of the obtained golf ball.

In case that the rubber composition does not contain the followingcomponents (d), the amount of (c) the crosslinking initiator ispreferably 0.1 part by mass or less with respect to 100 parts by mass of(a) the base rubber. If the amount of (c) the crosslinking initiator is0.1 part by mass or less with respect to 100 parts by mass of (a) thebase rubber, the crosslinking density is decreased for the amount of theco-crosslinking agent, the breaking strain is increased, and thedurability is enhanced.

The core rubber composition preferably contains at least one (d)additive selected from the group consisting of (d1) a hinderedphenol-based compound and (d2) a hindered amine-based compound. Thecomponent (d) lowers the crosslinking density difference of the corewhile keeping the hardness difference of the core to an extent which isnot lower than a predetermined level.

[(d1) Hindered Phenol-Based Compound]

The hindered phenol-based compound is a compound having a hydroxyphenylstructure with a hydroxy group thereof being sterically protected by abulky functional group. The bulky functional group preferably exists ata position adjacent to the hydroxy group. Examples of the bulkyfunctional group include t-butyl group, and a long chain alkyl groupoptionally having a part of the carbon atoms thereof replaced with asulfur atom. As the hindered phenol-based compound, a compound having atert-butylhydroxyphenyl structure with at least one tert-butyl group ispreferable, a compound having a di-tert-butylhydroxyphenyl structurewith two tert-butyl groups is more preferable.

Examples of the compound having the tert-butylhydroxyphenyl structurewith at least one tert-butyl group include compounds having a structuresuch as 3-tert-butyl-4-hydroxyphenyl or3,5-di-tert-butyl-4-hydroxyphenyl. Among them, the compound having3,5-di-tert-butyl-4-hydroxyphenyl structure is preferable.

Specific examples of the hindered phenol-based compound include acompound having one hydroxyphenyl structure, such as dibutylhydroxytoluene (BHT), 4,6-bis(octylthiomethyl)-o-cresol,4,6-bis[(dodecylthio)methyl]-o-cresol,2,4-dimethyl-6-(1-methylpentadecyl) phenol (e.g. Irganox 1141 availablefrom BASF Japan Ltd.), and octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate (e.g. ADK STAB AO-50 available from Adeka Corporation).

Other specific examples of the hindered phenol-based compound include acompound having two hydroxyphenyl structures, such as 2,2′-methylenebis(4-ethyl-6-tert-butylphenol) (e.g. YOSHINOX 425 available fromMitsubishi Chemical Corporation), 2,2′-methylenebis(4-methyl-6-tert-butylphenol) (e.g. Sandant 2246 available fromSanshin Chemical Industry Co., Ltd.), 4,4′-butylidenebis(3-methyl-6-tert-butylphenol) (e.g. YOSHINOX BB available fromMitsubishi Chemical Corporation),4,4′-thiobis(3-methyl-6-tert-butylphenol) (e.g. NOCRAC 300 availablefrom Ouchi Shinko Chemical Industrial Co., Ltd.), 4,4-methylenebis(2,6-di-tert-butylphenol),2,6-di-tert-butyl-4-({2-[(3,5-di-tert-butyl-4-hydroxyphenyl)sulfanyl]propane-2-yl}sulfanyl) phenol (probucol), and3,9-bis[2-[β-(3-tert-butyl-4-hydroxy methylphenyl)propionyloxy]-1,1-dimethylethyl]-2,4,8,10-tetraoxaspiro[5.5] undecane(e.g. ADK STAB AO-80 available from Adeka Corporation).

Other specific examples of the hindered phenol-based compound include acompound having three hydroxyphenyl structures, such as1,3,5-tris-(3,5-di-tert-butyl-4-hydroxybenzyl)-1,3-5-triazine-2,4,6(1H,3H,5H-)-trione(e.g. ADK STAB AO-20 available from Adeka Corporation), and1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl) benzene (e.g.ADK STAB AO330 available from Adeka Corporation).

Other specific examples of the hindered phenol-based compound include acompound having four hydroxyphenyl structures, such as pentaerythritoltetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate] (e.g. ADKSTAB AO-60 available from Adeka Corporation).

The hindered phenol-based compound may be used solely, or two or more ofthem may be used in combination.

As the hindered phenol-based compound, at least one compound selectedfrom the group consisting of dibutylhydroxy toluene,octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate,4,4′-methylene bis(2,6-di-tert-butylphenol),2,6-di-tert-butyl-4-({2-[(3,5-di-tert-butyl-4-hydroxyphenyl)sulfanyl]propane-2-yl}sulfanyl) phenol,1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione,and pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate is preferable.

As (d2) the hindered amine-based compound, a compound having2,2,6,6-tetramethyl-4-piperidyl group represented by the followingchemical formula (1) is preferable.

In the formula (1), R¹¹ is a hydrogen atom, a hydroxy group, an alkylgroup having 1 to 30 carbon atoms, a hydroxyalkyl group having 1 to 30carbon atoms, an alkoxy group having 1 to 30 carbon atoms, ahydroxyalkoxy group having 1 to 30 carbon atoms, or an oxyradical.

The hindered amine-based compound represented by the chemical formula(1) includes the hindered amine-based compound as represented by thechemical formula (2) or chemical formula (3).

The hindered amine-based compound represented by the following chemicalformula (2) is a so-called N-alkyl type hindered amine-based compound orNH type hindered amine-based compound.

In the formula (2), R¹² is a hydrogen atom, a hydroxy group, an alkylgroup having 1 to 30 carbon atoms, a hydroxyalkyl group having 1 to 30carbon atoms, or an oxyradical.

The hindered amine-based compound represented by the following chemicalformula (3) is a so-called N-alkoxy type hindered amine compound.

In the chemical formula (3), R¹² is an alkyl group having 1 to 30 carbonatoms or a hydroxyalkyl group having 1 to 30 carbon atoms.

Specific examples of (d2) the hindered amine-based compound includecompounds represented by the chemical formulae (4) to (6).

In the chemical formula (4), R¹⁴ and R¹⁵ are each independently ahydrogen atom, a hydroxy group, an alkyl group having 1 to 30 carbonatoms, a hydroxyalkyl group having 1 to 30 carbon atoms, an alkoxy grouphaving 1 to 30 carbon atoms, a hydroxyalkoxy group having 1 to 30 carbonatoms, or an oxyradical. R¹³ is an alkylene group having 1 to 20 carbonatoms.

In the chemical formula (5), R¹⁶ is a hydrogen atom, a hydroxy group, analkyl group having 1 to 30 carbon atoms, a hydroxyalkyl group having 1to 30 carbon atoms, an alkoxy group having 1 to 30 carbon atoms, ahydroxyalkoxy group having 1 to 30 carbon atoms, or an oxyradical. R¹⁷is an alkyl group having 1 to 30 carbon atoms, or an alkenyl grouphaving 2 to 30 carbon atoms.

In the chemical formula (6), R¹⁸ and R¹⁹ are each independently ahydrogen atom, a hydroxy group, an alkyl group having 1 to 30 carbonatoms, a hydroxyalkyl group having 1 to 30 carbon atoms, an alkoxy grouphaving 1 to 30 carbon atoms, a hydroxyalkoxy group having 1 to 30 carbonatoms, or an oxyradical.

Specific examples of (d2) the hindered amine-based compound include ADKSTAB LA-52 (tetrakis(1,2,2,6,6-pentamethyl-4-piperidyl)butane-1,2,3,4-tetracarboxylate), ADK STAB LA-57(tetrakis(2,2,6,6-tetramethyl-4-piperidyl)butane-1,2,3,4-tetracarboxylate), ADK STAB LA-63P, ADK STAB LA-68, ADKSTAB LA-72 (bis(1,2,2,6,6-pentamethyl-4-piperidyl) sebacate, ADK STABLA-77Y (bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate, and ADK STABLA-81 (bis(1-undecanoxy-2,2,6,6-tetramethylpiperidine-4-yl) carbonateavailable from Adeka Corporation.

Specific examples of (d2) the hindered amine-based compound include thefollowing products available from BASF Japan Ltd.

1) Chimassorb 2020FDL

1,6-Hexanediamine, N,N′-bis(2,2,6,6-tetramethyl-4-piperidinyl)-polymerwith 2,4,6-trichloro-1,3,5-triazine, reaction products withN-butyl-1-butanamine and N-butyl-2,2,6,6-tetramethyl-4-piperidinamine

2) Chimassorb 944FDL

Poly[[6-[(1,1,3,3-tetramethylbutyl)amino]-1,3,5-triazine-2,4-diyl][(2,2,6,6-tetramethyl-4-piperidinyl)imino]-1,6-hexanediyl[(2,2,6,6-tetramethyl-4-piperidinyl)imino]])

3) TINUVIN 622SF

Butanedioic acid, dimethylester, polymer with4-hydroxy-2,2,6,6-tetramethyl-1-piperidine ethanol)

4) TINUVIN PA144

Bis(1,2,2,6,6-pentamethyl-4-piperidinyl)-2-butyl-2-(4-hydroxy-3,5-di-tert-butylbenzyl)propanedioate

(d2) The hindered amine-based compound may be used solely, or two ormore of them may be used in combination. In addition, (d1) the hinderedphenol-based compound and (d2) the hindered amine-based compound may beused in combination.

In case that the core rubber composition contains (d1) the hinderedphenol-based compound and/or (d2) the hindered amine-based compound as(d) the additive, the amount of (d1) the hindered phenol-based compoundand/or (d2) the hindered amine-based compound is preferably 0.5 part bymass or more, more preferably 1 part by mass or more, and even morepreferably 1.5 parts by mass or more, and is preferably 15 parts by massor less, more preferably 13 parts by mass or less, and even morepreferably 11 parts by mass or less, with respect to 100 parts by massof (a) the base rubber. If the amount of (d1) the hindered phenol-basedcompound and/or (d2) the hindered amine-based compound falls within theabove range, the crosslinking density is low for the amount of theco-crosslinking agent, the strain at break is increased, and thedurability is enhanced.

[(e) Organic Sulfur Compound]

The core rubber composition preferably further contains (e) an organicsulfur compound. If (e) the organic sulfur compound is contained, theobtained core has further enhanced resilience.

As (e) the organic sulfur compound, at least one compound selected fromthe group consisting of thiols (thiophenols or thionaphthols),polysulfides, thiurams, thiocarboxylic acids, dithiocarboxylic acids,sulfenamides, dithiocarbamates, and thiazoles is preferable.

Examples of the thiols include thiophenols and thionaphthols. Examplesof the thiophenols include thiophenol; thiophenols substituted with afluoro group, such as 4-fluorothiophenol, 2,4-difluorothiophenol,2,5-difluorothiophenol, 2,6-difluorothiophenol,2,4,5-trifluorothiophenol, 2,4,5,6-tetrafluorothiophenol andpentafluorothiophenol; thiophenols substituted with a chloro group, suchas 2-chlorothiophenol, 4-chlorothiophenol, 2,4-dichlorothiophenol,2,5-dichlorothiophenol, 2,6-dichlorothiophenol,2,4,5-trichlorothiophenol, 2,4,5,6-tetrachlorothiophenol andpentachlorothiophenol; thiophenols substituted with a bromo group, suchas 4-bromothiophenol, 2,4-dibromothiophenol, 2,5-dibromothiophenol,2,6-dibromothiophenol, 2,4,5-tribromothiophenol,2,4,5,6-tetrabromothiophenol and pentabromothiophenol; thiophenolssubstituted with an iodo group, such as 4-iodothiophenol,2,4-diiodothiophenol, 2,5-diiodothiophenol, 2,6-diiodothiophenol,2,4,5-triiodothiophenol, 2,4,5,6-tetraiodothiophenol andpentaiodothiophenol; and metal salts thereof. As the metal salt, zincsalt is preferable.

Examples of the thionaphthols (naphthalenethiols) include2-thionaphthol, 1-thionaphthol, 1-chloro-2-thionaphthol,2-chloro-1-thionaphthol, 1-bromo-2-thionaphthol, 2-bromo-1-thionaphthol,1-fluoro-2-thionaphthol, 2-fluoro-1-thionaphthol,1-cyano-2-thionaphthol, 2-cyano-1-thionaphthol, 1-acetyl-2-thionaphthol,2-acetyl-1-thionaphthol, and metal salts thereof. Among them,2-thionaphthol, 1-thionaphthol, and metal salts thereof are preferable.As the metal salt, a divalent metal salt is preferable, zinc salt ismore preferable. Specific examples of the metal salt include zinc saltof 1-thionaphthol and zinc salt of 2-thionaphthol.

The polysulfides are organic sulfur compounds having a polysulfide bond,and examples thereof include disulfides, trisulfides, and tetrasulfides.As the polysulfides, diphenyl polysulfides are preferable.

Examples of the diphenyl polysulfides include diphenyl disulfide;diphenyl disulfides substituted with a halogen group, such asbis(4-fluorophenyl)disulfide, bis(2,5-difluorophenyl)disulfide,bis(2,6-difluorophenyl)disulfide, bis(2,4,5-trifluorophenyl)disulfide,bis(2,4,5,6-tetrafluorophenyl)disulfide,bis(pentafluorophenyl)disulfide, bis(4-chlorophenyl)disulfide,bis(2,5-dichlorophenyl)disulfide, bis(2,6-dichlorophenyl) disulfide,bis(2,4,5-trichlorophenyl) disulfide, bis(2,4,5,6-tetrachlorophenyl)disulfide, bis(pentachlorophenyl) disulfide, bis(4-bromophenyl)disulfide, bis(2,5-dibromophenyl) disulfide, bis(2,6-dibromophenyl)disulfide, bis(2,4,5-tribromophenyl) disulfide,bis(2,4,5,6-tetrabromophenyl) disulfide, bis(pentabromophenyl)disulfide, bis(4-iodophenyl) disulfide, bis(2,5-diiodophenyl) disulfide,bis(2,6-diiodophenyl) disulfide, bis(2,4,5-triiodophenyl) disulfide,bis(2,4,5,6-tetraiodophenyl) disulfide and bis(pentaiodophenyl)disulfide; and diphenyl disulfides substituted with an alkyl group, suchas bis(4-methylphenyl) disulfide, bis(2,4,5-trimethylphenyl) disulfide,bis(pentamethylphenyl) disulfide, bis(4-t-butylphenyl) disulfide,bis(2,4,5-tri-t-butylphenyl) disulfide, and bis(penta-t-butylphenyl)disulfide.

Examples of the thiurams include thiuram monosulfides such astetramethylthiuram monosulfide; thiuram disulfides such astetramethylthiuram disulfide, tetraethylthiuram disulfide andtetrabutylthiuram disulfide; and thiuram tetrasulfides such asdipentamethylenethiuram tetrasulfide. Examples of the thiocarboxylicacids include naphthalene thiocarboxylic acid. Examples of thedithiocarboxylic acids include naphthalene dithiocarboxylic acid.Examples of the sulfenamides include N-cyclohexyl-2-benzothiazolesulfenamide, N-oxydiethylene-2-benzothiazole sulfenamide, andN-t-butyl-2-benzothiazole sulfenamide.

As (e) the organic sulfur compound, the thiophenols and/or the metalsalts thereof, the thionaphthols and/or the metal salts thereof, thediphenyl disulfides, and the thiuram disulfides are preferable,2,4-dichlorothiophenol, 2,6-difluorothiophenol, 2,6-dichlorothiophenol,2,6-dibromothiophenol, 2,6-diiodothiophenol, 2,4,5-trichlorothiophenol,pentachlorothiophenol, 1-thionaphthol, 2-thionaphthol, diphenyldisulfide, bis(2,6-difluorophenyl) disulfide, bis(2,6-dichlorophenyl)disulfide, bis(2,6-dibromophenyl) disulfide, bis(2,6-diiodophenyl)disulfide, and bis(pentabromophenyl) disulfide are more preferable.

(e) The organic sulfur compound may be used solely, or two or more ofthem may be used in combination.

The amount of (e) the organic sulfur compound is preferably 0.05 part bymass or more, more preferably 0.1 part by mass or more, and even morepreferably 0.2 part by mass or more, and is preferably 5.0 parts by massor less, more preferably 3.0 parts by mass or less, and even morepreferably 2.0 parts by mass or less, with respect to 100 parts by massof (a) the base rubber. If the amount of (e) the organic sulfur compoundis less than 0.05 part by mass, the effect of adding (e) the organicsulfur compound may not be obtained, and the resilience of the golf ballmay not be enhanced. In addition, if the amount of (e) the organicsulfur compound is more than 5.0 parts by mass, the obtained golf ballhas a great compression deformation amount and thus the resiliencethereof may be lowered.

[(f) Metal Compound]

The core rubber composition preferably further contains (f) a metalcompound. (f) The metal compound can be used as, for example, a weightadjusting agent for the core rubber composition, or a neutralizing agentfor the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.

Examples of (f) the metal compound include a metal hydroxide such asmagnesium hydroxide, zinc hydroxide, calcium hydroxide, sodiumhydroxide, lithium hydroxide, potassium hydroxide, and copper hydroxide;a metal oxide such as magnesium oxide, calcium oxide, zinc oxide, andcopper oxide; and a metal carbonate such as magnesium carbonate, zinccarbonate, calcium carbonate, sodium carbonate, lithium carbonate, andpotassium carbonate. As (f) the metal compound, the divalent metalcompound is preferable, the zinc compound is more preferable. This isbecause the divalent metal compound reacts with the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms to form a metal crosslinking.In addition, if the zinc compound is used, the obtained golf ball hasbetter resilience.

(f) The metal compound may be used solely, or at least two of them maybe used in combination. In addition, the amount of (f) the metalcompound may be appropriately adjusted according to the desiredneutralization degree of (b) the α,β-unsaturated carboxylic acid having3 to 8 carbon atoms.

The core rubber composition may further contain an additive such as apigment, a filler for adjusting weight or the like, a peptizing agent,and a softener, where necessary.

The filler blended in the core rubber composition is mainly used as aweight adjusting agent for adjusting the weight of the golf ballobtained as a final product, and may be blended where necessary.Examples of the filler include an inorganic filler such as zinc oxide,barium sulfate, calcium carbonate, magnesium oxide, tungsten powder, andmolybdenum powder. As the filler, zinc oxide is particularly preferable.It is considered that zinc oxide acts as a vulcanizing aid to increasethe hardness of the core as a whole. The amount of the filler ispreferably 0.5 part by mass or more, more preferably 1 part by mass ormore, and is preferably 30 parts by mass or less, more preferably 25parts by mass or less, and even more preferably 20 parts by mass orless, with respect to 100 parts by mass of (a) the base rubber. If theamount of the filler is less than 0.5 part by mass, it is difficult toadjust the weight, and if the amount of the filler is more than 30 partsby mass, the weight proportion of the rubber component is decreased andthus the resilience tends to be lowered.

The amount of the peptizing agent is preferably 0.1 part by mass or moreand 5 parts by mass or less with respect to 100 parts by mass of (a) thebase rubber.

The golf ball according to the present disclosure comprises a sphericalcore and at least two cover layers covering the spherical core, whereinthe spherical core is formed from the above core rubber composition.

In the spherical core of the golf ball according to the presentdisclosure, the difference (core surface crosslinking density−corecenter crosslinking density) between the surface crosslinking density ofthe spherical core and the center crosslinking density of the sphericalcore is more than 1.0×10² mol/m³ and less than 9.0×10² mol/m³, and thehardness difference (Cs−Co) between the surface hardness Cs (Shore Chardness) of the spherical core and the center hardness Co (Shore Chardness) of the spherical core is 13.0 or more and 30.0 or less.

The golf ball according to the present disclosure is characterized inthat the crosslinking density difference between the core surface andthe core center is not higher than a predetermined value while thehardness difference between the core surface and the core center is notlower than a predetermined value. The golf ball having this core hasexcellent durability and has good shot feeling at the same time.

In the spherical core of the golf ball according to the presentdisclosure, the difference (core surface crosslinking density−corecenter crosslinking density) between the surface crosslinking density ofthe spherical core and the center crosslinking density of the sphericalcore is preferably more than 1.0×10² mol/m³, more preferably 1.5×10²mol/m³ or more, and even more preferably 2.0×10² mol/m³ or more, and ispreferably less than 9.0×10² mol/m³, more preferably 8.9×10² mol/m³ orless, and even more preferably 8.8×10² mol/m³ or less. If the difference(core surface crosslinking density−core center crosslinking density)between the surface crosslinking density of the spherical core and thecenter crosslinking density of the spherical core falls within the aboverange, good feeling is obtained while the durability is maintained.

The surface crosslinking density of the spherical core is preferably9.0×10² mol/m³ or more, more preferably 9.5×10² mol/m³ or more, and evenmore preferably 10.0×10² mol/m³ or more, and is preferably 22.0×10²mol/m³ or less, more preferably 21.0×10² mol/m³ or less, and even morepreferably 20.0×10² mol/m³ or less. If the surface crosslinking densityof the spherical core falls within the above range, the rubbercomposition is not excessively hard, and good feeling is obtained.

The center crosslinking density of the spherical core is preferably4.0×10² mol/m³ or more, more preferably 4.5×10² mol/m³ or more, and evenmore preferably 5.0×10² mol/m³ or more, and is preferably 13.0×10²mol/m³ or less, more preferably 12.0×10² mol/m³ or less, and even morepreferably 11.0×10² mol/m³ or less. If the center crosslinking densityof the spherical core falls within the above range, the rubbercomposition is not excessively soft, and the resilience can bemaintained.

The hardness difference (Cs−Co) between the surface hardness (Cs) andthe center hardness (Co) of the spherical core is preferably 13.0 ormore, more preferably 14.0 or more, and even more preferably 15.0 ormore, and is preferably 30.0 or less, more preferably 28.0 or less, andeven more preferably 26.0 or less in Shore C hardness. If the hardnessdifference (Cs−Co) between the surface hardness (Cs) and the centerhardness (Co) of the spherical core is 13.0 or more in Shore C hardness,the durability is maintained or enhanced. In addition, if the hardnessdifference (Cs−Co) between the surface hardness (Cs) and the centerhardness (Co) of the spherical core is 30.0 or less in Shore C hardness,the golf ball having further enhanced shot feeling on driver shots isobtained.

The surface hardness (Cs) of the spherical core is preferably 60.0 ormore, more preferably 65.0 or more, and even more preferably 70.0 ormore, and is preferably 90.0 or less, more preferably 88.0 or less, andeven more preferably 85.0 or less in Shore C hardness. If the surfacehardness (Cs) of the spherical core is 60.0 or more in Shore C hardness,the core has better resilience. In addition, the surface hardness (Cs)of the spherical core is 90.0 or less in Shore C hardness, the shotfeeling on driver shots is further enhanced.

The center hardness (Co) of the spherical core is preferably 30.0 ormore, more preferably 35.0 or more, and even more preferably 40.0 ormore in Shore C hardness. If the center hardness (Co) of the sphericalcore is 30.0 or more in Shore C hardness, the spherical core is notexcessively soft, and the resilience is better. In addition, the centerhardness (Co) of the spherical core is preferably 70.0 or less, morepreferably 68.0 or less, and even more preferably 67.0 or less in ShoreC hardness. If the center hardness (Co) is 70.0 or less in Shore Chardness, the core is not excessively hard, and the shot feeling isbetter.

The diameter of the spherical core of the golf ball according to thepresent disclosure is preferably 34.8 mm or more, more preferably 35.8mm or more, and even more preferably 36.8 mm or more, and is preferably42.2 mm or less, more preferably 41.8 mm or less, even more preferably41.2 mm or less, and most preferably 40.8 mm or less. If the diameter ofthe spherical core is 34.8 mm or more, the cover is not excessivelythick, and thus the resilience is better. On the other hand, if thediameter of the spherical core is 42.2 mm or less, the cover is notexcessively thin, and thus the cover functions better.

When the spherical core has the diameter in the range from 34.8 mm to42.2 mm, the compression deformation amount (shrinking amount along thecompression direction) of the spherical core when applying a load from98 N as an initial load to 1275 N as a final load to the spherical coreis preferably less than 3.8 mm, more preferably 3.7 mm or less, and evenmore preferably 3.6 mm or less, and is preferably 2.0 mm or more, morepreferably 2.3 mm or more, and even more preferably 2.5 mm or more. Ifthe compression deformation amount is less than 3.8 mm, the resilienceis better, and if the compression deformation amount is 2.0 mm or more,the shot feeling is better.

The spherical core of the golf ball according to the present disclosureis obtained by mixing and kneading the above core rubber composition andmolding the kneaded core rubber composition in a mold. The moldingconditions are not particularly limited, but the molding is generallycarried out at a temperature ranging from 130° C. to 200° C. under apressure ranging from 2.9 MPa to 11.8 MPa for 10 minutes to 60 minutes.For example, it is preferable that the heat pressing of the core rubbercomposition is preferably carried out at a temperature of 130° C. to200° C. for 10 to 60 minutes, or carried out in a two-step heating ofheating at a temperature of 130° C. to 150° C. for 20 to 40 minutesfollowed by heating at a temperature of 160° C. to 180° C. for 5 to 15minutes.

[Cover]

The golf ball according to the present disclosure comprises a sphericalcore and at least two cover layers covering the spherical core. The atleast two cover layers include a first cover layer and a second coverlayer positioned closer to the core than the first cover layer, anaverage hardness Dave=(Ti×Hi+To×Ho)/(Ti+To) of the first cover layer andthe second cover layer is 55 or more, where To (mm) is a thickness ofthe first cover layer, Ho (Shore D) is a slab hardness of the firstcover layer, Ti (mm) is a thickness of the second cover layer, and Hi(Shore D) is a slab hardness of the second cover layer.

The average hardness Dave of the first cover layer and the second coverlayer is preferably 56 or more, more preferably 57 or more. If theaverage hardness Dave is 55 or more, a good balance is stricken betweena high initial speed and a low spin rate on driver shots. The averagehardness Dave of the first cover layer and the second cover layer ispreferably 71 or less, more preferably 70 or less, and even morepreferably 69 or less. If the average hardness Dave is 71 or less, theshot feeling and the durability are better.

The second cover layer is not particularly limited, as long as thesecond cover layer is positioned closer to the core than the first coverlayer. For example, it is preferable that the first cover layer is anoutermost cover layer, and the second cover layer is an inner coverlayer adjacent to the outermost cover layer.

The slab hardness Hi of the second cover layer is preferably higher thanthe slab hardness Ho of the first cover layer. The slab hardnessdifference (Hi-Ho) between the second cover layer and the first coverlayer is preferably 2 or more, more preferably 4 or more, and even morepreferably 6 or more, and is preferably 30 or less, more preferably 28or less, and even more preferably 26 or less.

The slab hardness Ho (Shore D) of the first cover layer is preferably 32or more, more preferably 33 or more, and even more preferably 34 ormore, and is preferably 72 or less, more preferably 71 or less, and evenmore preferably 70 or less.

The thickness To of the first cover layer is preferably 0.3 mm or more,more preferably 0.4 mm or more, and even more preferably 0.5 mm or more,and is preferably 2.7 mm or less, more preferably 2.6 mm or less, andeven more preferably 2.5 mm or less.

The slab hardness Hi (Shore D) of the second cover layer is preferably32 or more, more preferably 33 or more, and even more preferably 34 ormore, and is preferably 72 or less, more preferably 71 or less, and evenmore preferably 70 or less.

The thickness Ti of the second cover layer is preferably 0.3 mm or more,more preferably 0.4 mm or more, and even more preferably 0.5 mm or more,and is preferably 2.7 mm or less, more preferably 2.6 mm or less, andeven more preferably 2.5 mm or less.

The total thickness of the cover is preferably 3.0 mm or less, morepreferably 2.9 mm or less, and even more preferably 2.8 mm or less. Ifthe total thickness of the cover is 3.0 mm or less, the obtained golfball has better resilience and shot feeling. The total thickness of thecover is preferably 1 mm or more, more preferably 1.1 mm or more, andeven more preferably 1.2 mm or more. If the total thickness of the coveris less than 1 mm, the durability or wear resistance of the cover may belowered.

The cover of the golf ball according to the present disclosure is formedfrom a cover composition containing a resin component. Examples of theresin component include an ionomer resin, a thermoplastic polyurethaneelastomer having a trade name of “Elastollan (registered trademark)”available from BASF Japan Ltd., a thermoplastic polyamide elastomerhaving a trade name of “Pebax (registered trademark)” available fromArkema K. K., a thermoplastic polyester elastomer having a trade name of“Hytrel (registered trademark)” available from Du Pont-Toray Co., Ltd.,and a thermoplastic styrene elastomer having a trade name of “TEFABLOC(registered trademark)” available from Mitsubishi Chemical Corporation.

Examples of the ionomer resin include a product prepared by neutralizingat least a part of carboxyl groups in a binary copolymer composed of anolefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atomswith a metal ion, a product prepared by neutralizing at least a part ofcarboxyl groups in a ternary copolymer composed of an olefin, anα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and anα,β-unsaturated carboxylic acid ester with a metal ion, and a mixturethereof. The olefin is preferably an olefin having 2 to 8 carbon atoms.Examples of the olefin include ethylene, propylene, butene, pentene,hexene, heptene, and octene. In particular, ethylene is preferable.Examples of the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms include acrylic acid, methacrylic acid, fumaric acid, maleic acidand crotonic acid. In particular, acrylic acid and methacrylic acid arepreferable. Examples of the α,β-unsaturated carboxylic acid esterinclude a methyl ester, an ethyl ester, a propyl ester, a n-butyl ester,an isobutyl ester of acrylic acid, methacrylic acid, fumaric acid, andmaleic acid. In particular, acrylic acid ester and methacrylic acidester are preferable. Among these, the ionomer resin preferably includesa metal ion-neutralized product of a binary copolymer composed ofethylene and (meth)acrylic acid, and/or a metal ion-neutralized productof a ternary copolymer composed of ethylene, (meth)acrylic acid and(meth)acrylic acid ester.

The cover composition for forming the cover of the golf ball accordingto the present disclosure preferably contains a thermoplasticpolyurethane elastomer or an ionomer resin as the resin component. It isalso preferred that when the ionomer resin is used, a thermoplasticstyrene elastomer is used in combination. The amount of the polyurethaneor ionomer resin in the resin component of the cover composition ispreferably 50 mass % or more, more preferably 60 mass % or more, andeven more preferably 70 mass % or more.

In addition to the resin component, the cover composition may furthercontain a pigment component such as a white pigment (e.g. titaniumoxide), a blue pigment and a red pigment, a weight adjusting agent suchas zinc oxide, calcium carbonate and barium sulfate, a dispersant, anantioxidant, an ultraviolet absorber, a light stabilizer, a fluorescentmaterial or fluorescent brightener, as long as they do not impair theperformance of the cover.

The amount of the white pigment (e.g. titanium oxide) is preferably 0.5part or more, more preferably 1 part or more, and is preferably 10 partsor less, more preferably 8 parts or less, with respect to 100 parts bymass of the resin component constituting the cover. If the amount of thewhite pigment is 0.5 part by mass or more, it is possible to impart theopacity to the resultant cover. In addition, if the amount of the whitepigment is more than 10 parts by mass, the durability of the resultantcover may deteriorate.

Examples of the method of molding the cover of the golf ball accordingto the present disclosure include a method which comprises molding thecover composition into a hollow shell, covering the core with aplurality of the hollow shells and performing compression molding(preferably a method which comprises molding the cover composition intoa hollow half-shell, covering the core with two of the half-shells andperforming compression molding); and a method which comprises injectionmolding the cover composition directly onto the core.

When molding the cover in a compression molding method, molding of thehalf shell is performed by either the compression molding method or theinjection molding method, and the compression molding method ispreferred. Compression molding the cover composition into a half shellis carried out, for example, under a pressure of 1 MPa or more and 20MPa or less at a temperature of −20° C. or more and 70° C. or lessrelative to the flow beginning temperature of the cover composition. Byperforming the molding under the above conditions, the half shell havinga uniform thickness is formed. Examples of the method for molding thecover by using the half shell include a method which comprises coveringthe core with two of the half shells and then performing compressionmolding. Compression molding half shells into the cover is carried out,for example, under a pressure of 0.5 MPa or more and 25 MPa or less at atemperature of −20° C. or more and 70° C. or less relative to the flowbeginning temperature of the cover composition. By performing themolding under the above conditions, the golf ball cover having a uniformthickness is formed.

In case of injection molding the cover composition into the cover, thecover composition extruded in a pellet form may be used for injectionmolding, or the cover materials such as the base resin components andthe pigment may be dry blended, followed by directly injection moldingthe blended materials. It is preferred to use upper and lower moldshaving a hemispherical cavity and pimples for forming the cover, whereina part of the pimples also serves as a retractable hold pin. Whenmolding the cover by injection molding, the hold pin is protruded tohold the core, the cover composition is charged and then cooled to formthe cover. For example, the cover composition heated at a temperatureranging from 200° C. to 250° C. is charged into a mold held under apressure of 9 MPa to 15 MPa for 0.5 to 5 seconds, and after cooling for10 to 60 seconds, the mold is opened to form the cover.

Concave portions called “dimples” are usually formed on the surface ofthe cover when the cover is molded. The total number of dimples formedon the cover is preferably 200 or more and 500 or less. If the totalnumber of dimples is less than 200, the dimple effect is hardlyobtained. On the other hand, if the total number of dimples exceeds 500,the dimple effect is hardly obtained because the size of the respectivedimples is small. The shape (shape in a plan view) of the dimples formedon the cover includes, without limitation, a circle; a polygonal shapesuch as a roughly triangular shape, a roughly quadrangular shape, aroughly pentagonal shape and a roughly hexagonal shape; and otherirregular shape. These shapes may be employed solely, or at least two ofthem may be employed in combination.

The golf ball body having the cover formed thereon is ejected from themold, and is preferably subjected to surface treatments such asdeburring, cleaning and sandblast where necessary. In addition, ifdesired, a paint film or a mark may be formed. The thickness of thepaint film is not particularly limited, and is preferably 5 μm or more,more preferably 7 μm or more, and is preferably 50 μm or less, morepreferably 40 μm or less, and even more preferably 30 μm or less. If thethickness of the paint film is less than 5 μm, the paint film is easy towear off due to the continued use of the golf ball, and if the thicknessof the paint film exceeds 50 μm, the dimple effect is reduced and thusthe flight performance of the golf ball may be lowered.

[Golf Ball]

The construction of the golf ball according to the present disclosure isnot particularly limited, as long as the golf ball comprises a sphericalcore and at least two cover layers covering the spherical core. TheFIGURE is a partially cutaway cross-sectional view of a golf ball 1according to one embodiment of the present disclosure. The golf ball 1comprises a spherical core 2, and a second cover layer 3 and a firstcover layer 4 covering the spherical core 2. The second cover layer 3 isdisposed closer to the core than the first cover layer. The first coverlayer 4 is an outermost cover layer. The second cover layer 3 is aninner cover layer adjacent to the outermost cover layer. A plurality ofdimples 31 are formed on the surface of the cover. Other portions thanthe dimples 31 on the surface of the golf ball 1 are lands 32. The golfball 1 is provided with a paint layer and a mark layer on an outer sideof the first cover layer 4, but these layers are not depicted.

The spherical core may have a single layered or multiple layeredconstruction, and preferably have the single layered construction.Unlike the multiple layered spherical core, the single layered sphericalcore does not have an energy loss at the interface of the multiplelayered spherical core when being hit, and thus has better resilience.In addition, the cover has a multiple layered construction composed ofat least two layers. Examples of the golf ball according to the presentdisclosure include a three-piece golf ball composed of a spherical core,an inner cover layer disposed around the spherical core, and anoutermost cover layer covering the inner cover layer; and a multi-piecegolf ball composed of a spherical core, and at least three cover layersdisposed around the spherical core. The present disclosure is suitablyapplied to any one of the above golf balls.

The golf ball according to the present disclosure preferably has adiameter ranging from 40 mm to 45 mm. In light of satisfying theregulation of US Golf Association (USGA), the diameter is mostpreferably 42.67 mm or more. In light of prevention of air resistance,the diameter is more preferably 44 mm or less, and most preferably 42.80mm or less. In addition, the golf ball according to the presentdisclosure preferably has a mass of 40 g or more and 50 g or less. Inlight of obtaining greater inertia, the mass is more preferably 44 g ormore, and most preferably 45.00 g or more. In light of satisfying theregulation of USGA, the mass is most preferably 45.93 g or less.

When the golf ball according to the present disclosure has a diameter ina range of from 40 mm to 45 mm, the compression deformation amount(shrinking amount along the compression direction) of the golf ball whenapplying a load from an initial load of 98 N to a final load of 1275 Nto the golf ball is preferably 1.7 mm or more, more preferably 1.9 mm ormore, and even more preferably 2.1 mm or more, and is preferably 3.4 mmor less, more preferably 3.2 mm or less, and even more preferably 3.0 mmor less. If the compression deformation amount is 1.7 mm or more, thegolf ball does not become excessively hard, and thus the shot feeling isbetter. On the other hand, if the compression deformation amount is 3.4mm or less, the resilience is better.

EXAMPLES

Next, the present disclosure will be described in detail by way ofexamples. However, the present disclosure is not limited to the examplesdescribed below. Various changes and modifications without departingfrom the spirit of the present disclosure are included in the scope ofthe present disclosure.

[Evaluation Method]

(1) Crosslinking Density (Mol/m³)

A circular slab having a thickness of 2 mm was cut out of the core in amanner that the circular slab passed through the geometry center of thecore. Then, samples having φ 3 mm were cut out of the above circularslab at the core center point and at a measuring point within 4 mminside from each part corresponding to the core surface, and the weightof the sample was measured with an electronic balance which ismeasurable to two decimal places (mg). The above sample and 8 ml oftoluene were added in a vial having a volume of 10 ml, and the vial wassealed with a plug, and allowed to stand for at least 72 hours. Then,the solution was discarded, and the weight of the sample after theimmersion was measured. The crosslinking density of the rubbercomposition was calculated based on the weights of the sample before andafter the swelling, using the following Flory-Rehner formula.v=−(In(1−v _(r))+v _(r) +Xv _(r) ²)/V _(S)(v _(r) ^(1/3) −v _(r)/2)

[v: crosslinking density, v_(r): volume proportion of rubber inswelling, X: interaction constant, V_(S): molar volume of toluene]v _(r) =V _(BR)/(V _(BR) +V _(T))V _(BR)=(w _(f) −w _(f) v _(f)/ρV _(T)=(w _(s) −w _(f)/ρ_(T)

[V_(BR): volume of BR (butadiene rubber) in rubber composition, V_(T):volume of swollen toluene, v_(f): weight proportion of filler in rubbercomposition, ρ: density of rubber composition, w_(f): weight of samplebefore immersion, w_(s): weight of sample after immersion, ρ_(T):density of toluene]

It is noted that in the calculation, V_(S) is 0.1063×10⁻³ m³/mol, ρ_(T)is 0.8669, and X is 0.47 according to the literature of Macromolecules2007, 40, 3669-3675.

(2) Compression Deformation Amount (Mm)

The deformation amount along the compression direction of the core orgolf ball (shrinking amount along the compression direction of the coreor golf ball), when applying a load from an initial load of 98 N to afinal load of 1275 N to the core or golf ball, was measured.

(3) Core Hardness (Shore C Hardness)

An automatic hardness tester Digitest II available from Bareiss companywas used to measure the hardness of the core. The Shore C hardnessmeasured on the surface of the core was adopted as the surface hardnessof the core. In addition, the core was cut into two equal hemispheres toobtain a cut plane, and the hardness at the central point of the cutplane was measured.

(4) Slab Hardness

Sheets with a thickness of about 2 mm were produced by injection moldingthe cover composition. The sheets were stored at a temperature of 23° C.for two weeks. At least three of these sheets were stacked on oneanother so as not to be affected by the measuring substrate on which thesheets were placed, and the hardness of the stack was measured with anautomatic hardness tester (Digitest II, available from Bareiss company)using a testing device of “Shore D”.

(5) Durability

A W #1 driver provided with a metal head (SRIXON ZX7 made in 2021, shafthardness: S, loft angle: 9.5°, available from Sumitomo RubberIndustries, Ltd.) was attached to a swing robot M/C available from GolfLaboratories, Inc. The golf ball was hit repeatedly until a crackoccurred, and the hitting times when the crack occurred were counted.The head speed hitting the golf ball was set to 50 m/sec for ahigh-skilled golfer. It is noted that the measurement was conductedusing twelve samples for each golf ball, and the average value thereofwas adopted as the hitting times for that golf ball. It is noted thatthe hitting times of the golf ball No. 16 were defined as 100, and thedurability of each golf ball was represented by converting the hittingtimes of each golf ball into this index and evaluated according to thefollowing standard.

-   -   E (Excellent): 115 or more    -   G (Good): 105 or more and less than 115    -   F (Fair): 95 or more and less than 105    -   P (Poor): less than 95        (6) Flight Distance

A W #1 driver provided with a metal head (SRIXON ZX7 made in 2021, shafthardness: S, loft angle: 9.5°, available from Sumitomo RubberIndustries, Ltd.) was attached to a swing robot M/C available from GolfLaboratories, Inc. The golf ball was hit and the total flight distancethereof was measured. The head speed hitting the golf ball was set to 50m/sec for a high-skilled golfer.

-   -   E (Excellent): 260 m or more    -   G (Good): 258 m or more and less than 260 m    -   F (Fair): 256 m or more and less than 258 m    -   P (Poor): less than 256 m        (7) Shot Feeling

An actual hitting test was carried out by twenty high-skilled golfersusing a driver. In accordance with the number of people who answered theshot feeling was soft, the golf balls were evaluated as follows.

Evaluation Standard

-   -   E (Excellent): 16 or more    -   G (Good): 10 or more and 15 or less    -   F (Fair): 3 or more and 9 or less    -   P (Poor): 2 or less        (8) Comprehensive Evaluation

A comprehensive evaluation regarding the durability, shot feeling andflight distance was done based on the following standard. It is notedthat the comprehensive evaluation B is an acceptable range.

-   -   A: The evaluation results of durability, shot feeling and flight        distance are only G (good) or E (excellent).    -   B: One of the evaluation results of durability, shot feeling and        flight distance is F (fair).    -   C: There is P (poor) or at least two F (fair) in the evaluation        results of durability, shot feeling and flight distance.        [Production of Golf Ball]        (1) Production of Core

According to the formulations shown in Table 1, the rubber compositionswere kneaded with a kneading roll, and heat-pressed in upper and lowermolds, each having a hemispherical cavity, at a predeterminedtemperature for 30 minutes to produce spherical cores having a diameterof 38.6 mm.

TABLE 1 Core formulation No. a b c d e f g Core BR730 100 100 100 100100 100 100 composition ZDA 37.3 36.6 36.6 36.6 35.5 34.8 27.8 (partsDicumyl 0.8 0.8 0.8 0.8 0.8 0.8 0.8 by mass) peroxide Zinc oxide 5 5 5 55 5 5 BHT 5 5 5 5 5 5 0.1 Benzoic acid 0 0 1 2 0 0 0 Barium sulfate *)*) *) *) *) *) *) Core formulation No. h i j k l m n Core BR730 100 100100 100 100 100 100 composition ZDA 27.8 30.7 34.4 44.1 51.4 58.7 29.0(parts Dicumyl 0.8 0.8 0.8 0.8 0.8 0.8 0.8 by mass) peroxide Zinc oxide5 5 5 5 5 5 5 BHT 0.5 1 3 10 15 20 0 Benzoic acid 0 0 0 0 0 0 0 Bariumsulfate *) *) *) *) *) *) *) *) An appropriate amount of barium sulfatewas added such that the golf ball had a mass of 45.50 g.

The materials used in Table 1 are shown as follows.

BR730: high-cis polybutadiene rubber (cis-1,4 bond amount=95 mass %,1,2-vinyl bond amount=1.3 mass %, Moony viscosity (ML₁₊₄(100° C.))=55,molecular weight distribution (Mw/Mn)=3) available from JSR Corporation

ZDA: zinc acrylate (surface treated with zinc stearate in an amount of10%) available from Nisshoku Techno Fine Chemical Co., Ltd.

Dicumyl peroxide: available from Tokyo Chemical Industry Co., Ltd.(one-minute half-life temperature: 175.2° C.)

Zinc oxide: WHITE SEAL available from INDOLYSAGHT Co. Ltd.

BHT: dibutylhydroxy toluene available from Tokyo Zairyo Co., Ltd.

Benzoic acid: available from Tokyo Chemical Industry Co., Ltd.

Barium sulfate: “Barium sulfate BD” available from Sakai ChemicalIndustry Co., Ltd.

(2) Preparation of Cover and Production of Golf Ball

According to the formulations shown in Table 2, the cover materials wereextruded with a twin-screw kneading type extruder to prepare the covercompositions in a pellet form. The conditions for extruding the covercompositions were a screw diameter of 45 mm, a screw rotational speed of200 rpm, and screw L/D=35, and the mixture was heated to 160 to 230° C.at the die position of the extruder. The obtained cover composition wasinjection molded onto the spherical core obtained above to produce golfballs (diameter: 42.70 mm, mass: 45.50 g) comprising the spherical coreand two cover layers covering the core. The first cover layer is anoutermost cover layer, and the second cover layer is an inner coverlayer adjacent to the outermost cover layer. Evaluation resultsregarding the obtained golf balls are summarized in Tables 3 to 4.

TABLE 2 Cover composition No. 1 2 3 4 5 6 Parts Himilan AM7337 26 2638.5 — — 50 by mass Himilan AM7329 26 40 38.5 61 55 50 Himilan 1555 — —— 37 45 — TEFABLOC T3221C 48 34 23 2 — — Titanium dioxide 4 4 4 4 4 4JF-90 0.2 0.2 0.2 0.2 0.2 0.2 Slab hardness (Shore D) 35 45 50 60 63 66

The materials used in Table 2 are shown as follows.

Himilan AM7337: Na neutralized ionomer available from Du Pont-MitsuiPolychemicals Co., Ltd.

Himilan AM7329: Zn neutralized ionomer available from Du Pont-MitsuiPolychemicals Co., Ltd.

Himilan 1555: Na neutralized ionomer available from Du Pont-MitsuiPolychemicals Co., Ltd.

TEFABLOC T3221C: styrene elastomer available from Mitsubishi ChemicalCorporation.

Titanium dioxide: available from Ishihara Sangyo Kaisha, Ltd.

JF-90: light stabilizer available from Johoku Chemical Co., Ltd.

TABLE 3 Golf ball No. 1 2 3 4 5 Core Core formulation a e f b bVulcanization temperature (° C.) 170 170 170 160 165 Compressiondeformation amount (mm) 2.60 3.60 3.80 3.10 3.20 Core center hardness(Co: Shore C) 58 58 58 63 60 Core surface hardness (Cs: Shore C) 80 8080 75 78 Hardness difference (Cs-Co: Shore C) 22 22 22 12 18 Core centercrosslinking density vo (×10² mol/m³) 9.3 9.3 9.3 9.3 9.3 Core surfacecrosslinking density vs (×10² mol/m³) 16.9 16.9 16.9 16.9 16.9Crosslinking density difference (vs-vo: ×10² mol/m³) 7.6 7.6 7.6 7.6 7.6Cover Second cover layer composition No. 6 6 6 6 6 Second cover layerthickness Ti (mm) 1.00 1.00 1.00 1.00 1.00 Second cover layer hardnessHi (Shore D) 66 66 66 66 66 First cover layer composition No. 3 3 3 3 3First cover layer thickness To (mm) 1.05 1.05 1.05 1.05 1.05 First coverlayer hardness Ho (Shore D) 50 50 50 50 50 Average hardness (Ti × Hi +To × Ho)/(Ti + To) 57.8 57.8 57.8 57.8 57.8 Golf ball Compressiondeformation amount (mm) 1.78 2.78 2.98 2.28 2.38 Durability E G P F GShot feeling F E E E E Flight distance E G F F G Comprehensiveevaluation B A C C A Golf ball No. 6 7 8 9 10 Core Core formulation c dg h i Vulcanization temperature (° C.) 170 170 170 170 170 Compressiondeformation amount (mm) 3.40 3.50 3.30 3.30 3.30 Core center hardness(Co: Shore C) 55 54 59 59 61 Core surface hardness (Cs: Shore C) 83 8580 80 80 Hardness difference (Cs-Co: Shore C) 28 31 21 21 19 Core centercrosslinking density vo (×10² mol/m³) 9.3 9.3 9.4 9.4 9.3 Core surfacecrosslinking density vs (×10² mol/m³) 16.9 16.9 23.2 23.2 18.1Crosslinking density difference (vs-vo: ×10² mol/m³) 7.6 7.6 13.8 13.88.8 Cover Second cover layer composition No. 6 6 6 6 6 Second coverlayer thickness Ti (mm) 1.00 1.00 1.00 1.00 1.00 Second cover layerhardness Hi (Shore D) 66 66 66 66 66 First cover layer composition No. 33 3 3 3 First cover layer thickness To (mm) 1.05 1.05 1.05 1.05 1.05First cover layer hardness Ho (Shore D) 50 50 50 50 50 Average hardness(Ti × Hi + To × Ho)/(Ti + To) 57.8 57.8 57.8 57.8 57.8 Golf ballCompression deformation amount (mm) 2.58 2.68 2.48 2.48 2.48 DurabilityE E F F G Shot feeling G F F F G Flight distance G F E E E Comprehensiveevaluation A C C C A

TABLE 4 Golf ball No. 11 12 13 14 15 16 Core Core formulation j b k l mn Vulcanization temperature (° C.) 170 170 170 170 170 170 Compressiondeformation amount (mm) 3.30 3.30 3.30 3.30 3.30 3.30 Core centerhardness (Co: Shore C) 60 58 55 55 55 59 Core surface hardness (Cs:Shore C) 80 80 78 74 70 80 Hardness difference (Cs-Co: Shore C) 20 22 2319 15 21 Core center crosslinking density vo (×10² mol/m³) 9.3 9.3 9.39.3 9.3 9.3 Core surface crosslinking density vs (×10² mol/m³) 18.0 16.913.0 12.0 10.0 23.2 Crosslinking density difference (vs-vo: ×10² mol/m³)8.7 7.6 3.7 2.7 0.7 13.9 Cover Second cover layer composition No. 6 6 66 6 6 Second cover layer thickness Ti (mm) 1.00 1.00 1.00 1.00 1.00 1.00Second cover layer hardness Hi (Shore D) 66 66 66 66 66 66 First coverlayer composition No. 3 3 3 3 3 3 First cover layer thickness To (mm)1.05 1.05 1.05 1.05 1.05 1.05 First cover layer hardness Ho (Shore D) 5050 50 50 50 50 Average hardness (Ti × Hi + To × Ho)/(Ti + To) 57.8 57.857.8 57.8 57.8 57.8 Golf ball Compression deformation amount (mm) 2.482.48 2.48 2.48 2.48 2.48 Durability E E E G P F Shot feeling E E G F F FFlight distance E E E E E E Comprehensive evaluation A A A B C C Golfball No. 17 18 19 20 21 Core Core formulation b b b b b Vulcanizationtemperature (° C.) 170 170 170 170 170 Compression deformation amount(mm) 3.30 3.30 3.30 3.30 3.30 Core center hardness (Co: Shore C) 58 5858 58 58 Core surface hardness (Cs: Shore C) 80 80 80 80 80 Hardnessdifference (Cs-Co: Shore C) 22 22 22 22 22 Core center crosslinkingdensity vo (×10² mol/m³) 9.3 9.3 9.3 9.3 9.3 Core surface crosslinkingdensity vs (×10² mol/m³) 16.9 16.9 16.9 16.9 16.9 Crosslinking densitydifference (vs-vo: ×10² mol/m³) 7.6 7.6 7.6 7.6 7.6 Cover Second coverlayer composition No. 6 6 6 6 3 Second cover layer thickness Ti (mm)1.00 1.00 1.00 1.00 1.00 Second cover layer hardness Hi (Shore D) 66 6666 66 50 First cover layer composition No. 1 2 4 5 5 First cover layerthickness To (mm) 1.05 1.05 1.05 1.05 1.05 First cover layer hardness Ho(Shore D) 35 45 60 63 63 Average hardness (Ti × Hi + To × Ho)/(Ti + To)50.1 55.2 62.9 64.5 56.7 Golf ball Compression deformation amount (mm)2.58 2.53 2.42 2.40 2.75 Durability E E E G F Shot feeling E E G F GFlight distance P F E E E Comprehensive evaluation C B A B B

It can be seen from Tables 3 and 4 that the golf ball according to thepresent disclosure has excellent durability and great flight distanceand good shot feeling for a golfer with a high head speed.

The present disclosure is suitably used as a golf ball.

The present disclosure (1) is a golf ball comprising a spherical coreand at least two cover layers covering the spherical core, wherein thespherical core is formed from a core rubber composition containing (a) abase rubber, (b) an α,β-unsaturated carboxylic acid and/or a metal saltthereof as a co-crosslinking agent, and (c) a crosslinking initiator, adifference (core surface crosslinking density−core center crosslinkingdensity) between a surface crosslinking density of the spherical coreand a center crosslinking density of the spherical core is more than1.0×10² mol/m³ and less than 9.0×10² mol/m³, a hardness difference(Cs−Co) between a surface hardness Cs (Shore C hardness) of thespherical core and a center hardness Co (Shore C hardness) of thespherical core is 13.0 or more and 30.0 or less, a compressiondeformation amount of the spherical core when applying a load from aninitial load of 98 N to a final load of 1275 N to the spherical core isless than 3.8 mm, and the at least two cover layers include a firstcover layer and a second cover layer positioned closer to the sphericalcore than the first cover layer, an average hardnessDave=(Ti×Hi+To×Ho)/(Ti+To) of the first cover layer and the second coverlayer is 55 or more, where To (mm) is a thickness of the first coverlayer, Ho (Shore D) is a slab hardness of the first cover layer, Ti (mm)is a thickness of the second cover layer, and Hi (Shore D) is a slabhardness of the second cover layer.

The present disclosure (2) is the golf ball according to the presentdisclosure (1), wherein the slab hardness Hi of the second cover layeris higher than the slab hardness Ho of the first cover layer.

The present disclosure (3) is the golf ball according to the presentdisclosure (1) or (2), wherein the cover has a total thickness in arange from 1.0 mm to 3.0 mm.

The present disclosure (4) is the golf ball according to any one of thepresent disclosures (1) to (3), wherein the surface crosslinking densityof the spherical core is 9.0×10² mol/m³ or more and 22.0×10² mol/m³ orless.

The present disclosure (5) is the golf ball according to any one of thepresent disclosures (1) to (4), wherein the center crosslinking densityof the spherical core is 4.0×10² mol/m³ or more and 13.0×10² mol/m³ orless.

The present disclosure (6) is the golf ball according to any one of thepresent disclosures (1) to (5), wherein the surface hardness Cs of thespherical core is 60.0 or more and 90.0 or less in Shore C hardness.

The present disclosure (7) is the golf ball according to any one of thepresent disclosures (1) to (6), wherein the center hardness Co of thespherical core is 30.0 or more and 70.0 or less in Shore C hardness.

The present disclosure (8) is the golf ball according to any one of thepresent disclosures (1) to (7), wherein the core rubber compositioncontains at least one (d) additive selected from the group consisting of(d1) a hindered phenol-based compound and (d2) a hindered amine-basedcompound.

The present disclosure (9) is the golf ball according to any one of thepresent disclosures (1) to (8), wherein the core rubber compositioncontains (d1) a hindered phenol-based compound and/or (d2) a hinderedamine-based compound in an amount ranging from 0.5 part by mass to 15parts by mass with respect to 100 parts by mass of (a) the base rubber.

The present disclosure (10) is the golf ball according to the presentdisclosure (8) or (9), wherein (d1) the hindered phenol-based compoundis at least one compound selected from the group consisting ofdibutylhydroxy toluene, octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 4,4′-methylene bis(2,6-di-tert-butylphenol),2,6-di-tert-butyl-4-({2-[(3,5-di-tert-butyl-4-hydroxyphenyl)sulfanyl]propane-2-yl}sulfanyl) phenol,1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione,and pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate.

The present disclosure (11) is the golf ball according to any one of thepresent disclosures (1) to (10), wherein the first cover layer is anoutermost cover layer, and the second cover layer is an inner coverlayer adjacent to the outermost cover layer.

This application is based on Japanese Patent application No. 2021-153491filed on Sep. 21, 2021, the content of which is hereby incorporated byreference.

The invention claimed is:
 1. A golf ball comprising a spherical core andat least two cover layers covering the spherical core, wherein thespherical core is formed from a core rubber composition containing (a) abase rubber, (b) an α,β-unsaturated carboxylic acid and/or a metal saltthereof as a co-crosslinking agent, and (c) a crosslinking initiator, adifference (core surface crosslinking density−core center crosslinkingdensity) between a surface crosslinking density of the spherical coreand a center crosslinking density of the spherical core is more than1.0×10² mol/m³ and 8.8×10² mol/m³ or less, a hardness difference (Cs−Co)between a surface hardness Cs (Shore C hardness) of the spherical coreand a center hardness Co (Shore C hardness) of the spherical core is13.0 or more and 30.0 or less, a compression deformation amount of thespherical core when applying a load from an initial load of 98 N to afinal load of 1275 N to the spherical core is less than 3.8 mm, and theat least two cover layers include a first cover layer and a second coverlayer positioned closer to the spherical core than the first coverlayer, an average hardness Dave=(Ti×Hi+To×Ho)/(Ti+To) of the first coverlayer and the second cover layer is 55 or more, where To (mm) is athickness of the first cover layer, Ho (Shore D) is a slab hardness ofthe first cover layer, Ti (mm) is a thickness of the second cover layer,and Hi (Shore D) is a slab hardness of the second cover layer.
 2. Thegolf ball according to claim 1, wherein the slab hardness Hi of thesecond cover layer is higher than the slab hardness Ho of the firstcover layer.
 3. The golf ball according to claim 2, wherein a hardnessdifference (Hi−Ho) between the slab hardness Hi of the second coverlayer and the slab hardness Ho of the first cover layer is 2 or more and30 or less in Shore D hardness.
 4. The golf ball according to claim 1,wherein the cover has a total thickness in a range from 1.0 mm to 3.0mm.
 5. The golf ball according to claim 1, wherein the surfacecrosslinking density of the spherical core is 9.0×10² mol/m³ or more and22.0×10² mol/m³ or less.
 6. The golf ball according to claim 1, whereinthe center crosslinking density of the spherical core is 4.0×10² mol/m³or more and 13.0×10² mol/m³ or less.
 7. The golf ball according to claim1, wherein the surface hardness Cs of the spherical core is 60.0 or moreand 90.0 or less in Shore C hardness, and the center hardness Co of thespherical core is 30.0 or more and 70.0 or less in Shore C hardness. 8.The golf ball according to claim 1, wherein the difference (core surfacecrosslinking density−core center crosslinking density) is more than1.0×10² mol/m³ and 7.6×10² mol/m³ or less.
 9. The golf ball according toclaim 1, wherein the core rubber composition contains at least one (d)additive selected from the group consisting of (d1) a hinderedphenol-based compound and (d2) a hindered amine-based compound.
 10. Thegolf ball according to claim 9, wherein (d1) the hindered phenol-basedcompound includes at least one compound selected from the groupconsisting of dibutylhydroxy toluene,octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate,4,4′-methylene bis(2,6-di-tert-butylphenol),2,6-di-tert-butyl-4-({2-[(3,5-di-tert-butyl-4-hydroxyphenyl)sulfanyl]propane-2-yl}sulfanyl) phenol,1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione, and pentaerythritoltetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate.
 11. The golfball according to claim 1, wherein the core rubber composition contains(d1) a hindered phenol-based compound and/or (d2) a hindered amine-basedcompound in an amount ranging from 0.5 part by mass to 15 parts by masswith respect to 100 parts by mass of (a) the base rubber.
 12. The golfball according to claim 1, wherein the first cover layer is an outermostcover layer, and the second cover layer is an inner cover layer adjacentto the outermost cover layer.
 13. The golf ball according to claim 1,wherein the average hardness Dave of the first cover layer and thesecond cover layer is 55 or more and 71 or less.
 14. The golf ballaccording to claim 1, wherein the slab hardness Ho of the first coverlayer is 32 or more and 72 or less in Shore D hardness, and the slabhardness Hi of the second cover layer is 32 or more and 72 or less inShore D hardness.
 15. The golf ball according to claim 1, wherein thecompression deformation amount of the spherical core is 2.0 mm or moreand less than 3.8 mm.
 16. The golf ball according to claim 1, whereinthe thickness To of the first cover layer is 0.3 mm or more and 2.7 mmor less, and the thickness Ti of the second cover layer is 0.3 mm ormore and 2.7 mm or less.
 17. The golf ball according to claim 1, whereinthe core rubber composition contains (d1) a hindered phenol-basedcompound and/or (d2) a hindered amine-based compound in an amountranging from 10 parts by mass to 15 parts by mass with respect to 100parts by mass of (a) the base rubber.
 18. The golf ball according toclaim 1, wherein the core rubber composition contains at least one (d)additive selected from the group consisting of (d1) a hinderedphenol-based compound and (d2) a hindered amine-based compound, andcontains (c) the crosslinking initiator in an amount ranging from 0.2part by mass to 5.0 parts by mass with respect to 100 parts by mass of(a) the base rubber.
 19. The golf ball according to claim 1, wherein thecore rubber composition neither contains (d1) a hindered phenol-basedcompound nor (d2) a hindered amine-based compound, and contains (c) thecrosslinking initiator in an amount of 0.1 part by mass or less withrespect to 100 parts by mass of (a) the base rubber.
 20. The golf ballaccording to claim 1, wherein the core rubber composition consists of:(a) the base rubber, (b) the α,β-unsaturated carboxylic acid and/or themetal salt thereof as the co-crosslinking agent, (c) the crosslinkinginitiator, and at least one optional component selected from the groupconsisting of (d1) a hindered phenol-based compound, (d2) a hinderedamine-based compound, (e) an organic sulfur compound, (f) a metalcompound selected from a metal hydroxide, a metal oxide and a metalcarbonate, a pigment, a filler for adjusting weight, a peptizing agent,and a softener.